Abstract
Abstract Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off limb in the Hβ and Ca ii 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in Hβ line-wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the Hβ wing and the nominal photospheric limb is about 300–500 km. The Ca ii 8542 Å line-wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, toward the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb Hβ ribbons and their significant elevation above the photosphere. The ribbons in the Ca ii 8542 Å line-wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the Hβ line-wing intensities can be a useful tracer of flare energy deposition in the lower solar atmosphere.
Highlights
Solar flares are phenomena associated with sudden energy releases caused by the rapid reconfiguration of coronal magnetic fields
The off-limb brightening located in the ranges x = 10–22 Mm and y = 0–7 Mm in Figure 2 cannot be confidently identified as the footpoints of a flare loop arcade
Far-wing flare intensities of Hβ and Ca II lines can be considered as emission from an optically thin plasma, they are less attenuated by the overlying atmosphere along the line of sight (LoS) (Asai et al 2012)
Summary
Solar flares are phenomena associated with sudden energy releases caused by the rapid reconfiguration of coronal magnetic fields. Dynamic flare models obtained by radiation-hydrodynamic (RHD) simulations such as RADYN (Carlsson & Stein 1997; Abbett & Hawley 1999; Allred et al 2015) produce a strong increase of wing emission in the Balmer lines as a response to electron beam heating (Kuridze et al 2015; Rubio da Costa et al 2015). These results suggest that Balmer line-wing emission can be a very effective tracer of flare energy deposition in the lower atmosphere (Canfield & Gayley 1987). The RHD code RADYN is used to investigate the formation of the Hβ and Ca II 8542 Å line profiles and their intensity contribution functions in the flaring atmosphere involving ribbons at the limb
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